Solvent nucleophilicities of hexafluoroisopropanol/water mixtures

Article abstract of DOI:10.1002/poc.3064

First-order rate constants k₁ for the trapping of various donor- and acceptor-substituted benzhydrylium ions in mixtures of 1,1,1,3,3,3- hexafluoro-2-propanol (HFIP) and water ranging from 50 to 99% HFIP (w/w) were determined by laser flash pho

Full text of DOI:10.1002/poc.3064

Research Article
Received: 7 August 2012,
Revised: 12 September 2012,
Accepted: 21 September 2012,
Published online in Wiley Online Library: 26 November 2012
(wileyonlinelibrary.com) DOI: 10.1002/poc.3064
Solvent nucleophilicities of
hexafluoroisopropanol/water mixtures
Johannes Ammera and Herbert Mayra*
First-order rate constants k1 for the trapping of various donor- and acceptor-substituted benzhydrylium ions in
mixtures of 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and water ranging from 50 to 99% HFIP (w/w) were determined
by laser flash photolytic generation of benzhydrylium ions from benzhydryl triarylphosphonium salts in these solvents.
From these rate constants, we derived the solvent-specific reactivity parameters N1 and sN for HFIP/water mixtures as
defined by the linear free energy relationship lg k1(20 ꢀC)= sN(N1 + E). Copyright © 2012 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this paper.
INTRODUCTION
RESULTS AND DISCUSSION
Because of their unique solvating properties, 1,1,1,3,3,3-hexafluoro-
2-propanol (HFIP) and HFIP/water mixtures have been employed
as solvents for synthetic transformations[1–7] and for the study of
protein structures.[8,9] HFIP/water mixtures have also played a key
role in solvolytic studies due to their high ionizing powers and
low nucleophilicities,[10–14] which allowed one to investigate SN1
reactions of substrates which solvolyze with nucleophilic solvent
assistance in other media.[15] However, also in HFIP/water mixtures,
a change from SN1 to SN2 processes is occurring, when the SN1
process would lead to poorly stabilized carbocations. In order to
predict this change of mechanism from SN1 to SN2, the nucleophilic
reactivity of the solvent must be known.[16]
Irradiation of ca. 10ꢁ4 M solutions of the phosphonium salts E(1–13)–
PAr+3 BF–4 (Ar = Ph or p-Cl-C6H4) in mixtures of HFIP and water
(W) ranging from 50HFIP50W to 99HFIP1W (w/w) yielded the
benzhydrylium ions E(1–13)+ (Scheme 1 and Table 1).
In those cases where the reactions of E(1–13)+ with the solvent
were faster than the recombination with the photo-leaving
group PAr3, exponential decays of the UV/Vis absorbances of
E(1–13)+ can be observed, as illustrated in Fig. 1a for the decay
of the most electrophilic carbocation in our series, E13+, in
99HFIP1W. From the exponential decays, we derived the first-order
rate constants k1 (sꢁ1) listed in Table 2 for the reactions of E+ with
the HFIP/water mixtures.
We have recently reported that highly reactive acceptor-
substituted benzhydrylium ions can be generated by laser flash
photolysis of phosphonium salts[17] and employed this method
to determine the empirical electrophilicity parameters E of these
benzhydrylium ions[18] as defined by Eqn 1. This linear free energy
relationship relates the second-order rate constants k2 (Mꢁ1 sꢁ1) of
bimolecular reactions of electrophiles with nucleophiles to the
electrophile-specific parameter E and the nucleophile-specific
parameters N and sN.[19–21]
Reactions which proceed with rate constants lower than ca.
104 sꢁ1 were not investigated, because the kinetics of these
reactions are complicated by the recombination of E+ with the
photo-leaving group PAr3 which proceeds on a similar time
scale.[17] To determine the rate constants of slower reactions,
one might employ anionic photo-leaving groups such as
p-cyanophenolate, which can be expected to undergo significantly
slower recombination reactions with the photogenerated
carbocations due to the exceptionally good solvation of anions
by fluorinated alcohols.[23] Since a major goal of this investigation
was the characterization of the electrophilic reactivities of
acceptor-substituted benzhydrylium ions towards relatively weak
nucleophiles such as HFIP/water mixtures, we have not included
p-cyanophenolates in this study.
lg k2ð20 ꢀCÞ ¼ sNðN þ EÞ
(1)
Plots of lg k1 for the reactions of E+ with the HFIP/water mixtures
versus the E parameters of E+ were linear (Fig. 2) and allowed us
to derive the solvent nucleophilicity parameters N1 and sN listed
in Table 2. Figure 2 illustrates the excellent correlations of k1 with-
the E parameters that were derived from reactions of E+ with
Equation 1 can also be used to predict the first-order rate
constants k1 (sꢁ1) for the reactions of electrophiles with solvents,
when the solvent-specific parameters N1 and sN are employed
(Eqn 1a).[22]
lg k1ð20 ꢀCÞ ¼ sNðN1 þ EÞ
(1a)
* Correspondence to: Herbert Mayr, Department Chemie, Ludwig-Maximilians-
Universität München, Butenandtstrasse 5–13 (Haus F), 81377 München, Germany.
E-mail: Herbert.Mayr@cup.uni-muenchen.de
With the aid of our recently determined electrophilicity
parameters E for the highly reactive acceptor-substituted
benzhydrylium ions,[18] we can now provide quantitative reactivity
parameters for the weakly nucleophilic HFIP/water mixtures.
a J. Ammer, H. Mayr
Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstrasse
5-13 (Haus F), 81377 München, Germany
J. Phys. Org. Chem. 2013, 26 59–63
Copyright © 2012 John Wiley & Sons, Ltd.

J. AMMER AND H. MAYR
p-nucleophiles in CH2Cl2. Only very small deviations are
found for the most electrophilic benzhydrylium ion in the
series, E13+, although the electrophilicity parameter of this
carbocation had been derived from only one rate constant
because of its extremely high reactivity.[18] The kinetic data
from this work thus corroborate the electrophilicity parameter
E = 8.02[18] for E13+.
for the reaction of E8+ with HFIP/water mixtures of varying
water content, which is similar to a N1 versus % H2O plot as the
sN parameters are closely similar.
It has been reported that the microscopic structures of HFIP/
water mixtures change when the water content is varied. The
structures of the solvent mixtures investigated in this work
range from micelle-like assemblies of HFIP molecules in water
(50HFIP50W; 38.5 vol.-% HFIP, xHFIP = 0.097) over a poorly
characterized complex intermediate structure of small HFIP
associates (70HFIP30W to 95HFIP5W; 59–92 vol.-% HFIP,
0.200 ≤ xHFIP ≤ 0.671) to short helical chains of HFIP molecules
as in neat HFIP (97HFIP3W to 99HFIP1W; >95 vol.-% HFIP,
xHFIP ≥ 0.776).[24] Despite these large structural variations, lg k1
for the reactions of E8+ with the HFIP/water mixtures increases
almost linearly with the molar fraction of water over the entire
range from 99HFIP1W to 50HFIP50W (Fig. 3b). A similar effect
The 4,4-dichlorobenzhydrylium ion E7+ deviates slightly from
these correlations. Although E7+ has the same electrophilicity
as E6+ in CH2Cl2, the rate constants for reactions of E7+ in trifluor-
oethanol and HFIP/water mixtures are lower than those of E6+ by
a factor of 1.5 to 2.2 (Fig. 2).
The nucleophilic reactivities of the HFIP/water mixtures increase
with increasing water content, which is particularly pronounced
for the mixtures with low water content. Figure 3 compares lg k1
Figure 1. (a) Decay of the absorbance of E13+ at l = 439 nm observed
after irradiation of a 8.52 ꢂ 10ꢁ5 M solution of E13–P(p-Cl-C6H4)3+ BF–4 in
99HFIP1W and exponential fit of the data (k1 = 5.11 ꢂ 106 sꢁ1
,
R2 = 0.9685). (b) Plot of lg k1 obtained from reactions of E8+, E11+,
E12+, and E13+ with 99HFIP1W against the electrophilicity parameters
E of these benzhydrylium ions (lg k1 = 1.09(ꢁ1.93 + E), R2 = 0.9973)
Scheme 1. Generation of benzhydrylium ions E+ by laser flash irradiation
of the phosphonium salts E–PAr+3 BF4– (Ar = Ph or p-Cl-C6H4)
Table 1. Electrophiles E(1–13)+ and their electrophilicity parameters E
no.
Ea
X
Y
E1+
E2+
4-MeO
4-Me
4-Me
4-F
3-F, 4-Me
H
H
4-Me
H
4-F
3-F, 4-Me
H
4-Cl
H
H
2.11
3.63
4.43
5.01
5.24
5.47
5.48
6.23
6.70
6.74
6.87
7.52
(8.02)b
E3+
E4+
E5+
E6+
E7+
4-Cl
3-F
E8+
E9+
4-(CF3)
3,5-F2
3-F
3,5-F2
3,5-F2
E10+
E11+
E12+
E13+
H
3-F
3-F
3,5-F2
aFrom ref.[18]
bApproximate value.
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Copyright © 2012 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2013, 26 59–63

SOLVENT NUCLEOPHILICITIES OF HEXAFLUOROISOPROPANOL/WATER MIXTURES
Table 2. First-order rate constants k1 (sꢁ1) for reactions of electrophiles E+ with HFIP/water mixtures and comparison with rate
constants kcalc (sꢁ1) calculated from Eqn 1a
Nucleophile
solventa
Electrophile
abbreviation
Experiment
k1 /sꢁ1
Calculated
kcalcd/sꢁ1
E+
Eb
kcalc/k1
c
N1, sN
50HFIP50W
38.5% (v/v)
xHFIP = 0.097
N1 = 1.50
sN = 1.03
E1+
E2+
ani(Ph)CH+
(tol)2CH+
2.11
3.63
4.43
5.01
3.63
4.43
5.01
5.24
5.47
4.43
5.01
5.47
6.23
6.74
6.87
4.43
5.01
5.47
5.48
6.23
6.70
6.87
7.52
4.43
5.01
5.47
6.23
6.87
7.52
5.01
5.47
5.48
6.23
6.74
6.87
7.52
5.01
5.47
5.48
6.23
6.87
7.52
(8.02)f
6.23
6.87
7.52
(8.02)f
5.56 ꢂ 103 e
1.50 ꢂ 105
1.20 ꢂ 106
5.32 ꢂ 106
1.13 ꢂ 105
8.06 ꢂ 105
3.25 ꢂ 106
3.32 ꢂ 106
7.52 ꢂ 106
1.10 ꢂ 105
3.74 ꢂ 105
1.01 ꢂ 106
5.21 ꢂ 106
1.55 ꢂ 107
2.10 ꢂ 107
3.88 ꢂ 104
1.39 ꢂ 105
3.99 ꢂ 105
2.61 ꢂ 105
1.96 ꢂ 106
6.26 ꢂ 106
9.10 ꢂ 106
2.92 ꢂ 107
1.67 ꢂ 104
5.78 ꢂ 104
1.70 ꢂ 105
8.61 ꢂ 105
4.53 ꢂ 106
1.55 ꢂ 107
1.52 ꢂ 104
5.01 ꢂ 104
3.00 ꢂ 104
2.68 ꢂ 105
1.11 ꢂ 106
1.46 ꢂ 106
6.48 ꢂ 106
6.17 ꢂ 103
2.04 ꢂ 104
1.24 ꢂ 104
1.14 ꢂ 105
6.54 ꢂ 105
2.60 ꢂ 106
1.20 ꢂ 107
5.18 ꢂ 104
2.72 ꢂ 105
1.15 ꢂ 106
5.11 ꢂ 106
5.23 ꢂ 103
1.92 ꢂ 105
1.28 ꢂ 106
5.07 ꢂ 106
1.17ꢂ 105
6.87 ꢂ 105
2.48 ꢂ 106
4.12 ꢂ 106
6.84 ꢂ 106
1.03 ꢂ 105
3.57 ꢂ 105
9.55 ꢂ 105
4.86 ꢂ 106
1.45 ꢂ 107
1.91 ꢂ 107
3.79 ꢂ 104
1.37 ꢂ 105
3.78 ꢂ 105
3.87 ꢂ 105
2.03 ꢂ 106
5.73 ꢂ 106
8.35 ꢂ 106
3.51 ꢂ 107
1.59 ꢂ 104
5.79 ꢂ 104
1.62 ꢂ 105
8.83 ꢂ 105
3.69 ꢂ 106
1.58 ꢂ 107
1.34 ꢂ 104
4.19 ꢂ 104
4.30 ꢂ 104
2.77 ꢂ 105
9.86 ꢂ 105
1.36 ꢂ 106
6.86 ꢂ 106
5.36 ꢂ 103
1.72 ꢂ 104
1.76 ꢂ 104
1.18 ꢂ 105
5.96 ꢂ 105
3.09 ꢂ 106
1.10 ꢂ 107
4.86 ꢂ 104
2.42 ꢂ 105
1.24 ꢂ 106
4.35 ꢂ 106
0.94
1.28
1.07
0.95
1.04
0.85
0.76
1.24
0.91
0.94
0.95
0.95
0.93
0.93
0.91
0.98
0.98
0.95
1.48
1.04
0.92
0.92
1.20
0.95
1.00
0.95
1.03
0.81
1.02
0.88
0.84
1.43
1.04
0.89
0.93
1.06
0.87
0.84
1.42
1.03
0.91
1.19
0.91
0.94
0.89
1.08
0.85
E3+
tol(Ph)CH+
(pfp)2CH+
(tol)2CH+
E4+
70HFIP30W
59.3% (v/v)
xHFIP = 0.200
N1 = 1.65
sN = 0.96
E2+
E3+
tol(Ph)CH+
(pfp)2CH+
–
E4+
E5+
E6+
(Ph)2CH+
90HFIP10W
84.9% (v/v)
xHFIP = 0.491
N1 = 0.96
sN = 0.93
E3+
tol(Ph)CH+
(pfp)2CH+
(Ph)2CH+
E4+
E6+
E8+
mfp(Ph)CH+
dfp(Ph)CH+
(mfp)2CH+
tol(Ph)CH+
(pfp)2CH+
(Ph)2CH+
E10+
E11+
E3+
93HFIP7W
89.3% (v/v)
xHFIP = 0.588
N1 = 0.34
sN = 0.96
E4+
E6+
E7+
(pcp)2CH+
mfp(Ph)CH+
tfm(Ph)CH+
(mfp)2CH+
dfp(mfp)CH+
tol(Ph)CH+
(pfp)2CH+
(Ph)2CH+
E8+
E9+
E11+
E12+
E3+
95HFIP5W
92.2% (v/v)
xHFIP = 0.671
N1 = ꢁ0.10
sN = 0.97
E4+
E6+
E8+
mfp(Ph)CH+
(mfp)2CH+
dfp(mfp)CH+
(pfp)2CH+
(Ph)2CH+
E11+
E12+
E4+
97HFIP3W
95.3% (v/v)
xHFIP = 0.776
N1 = ꢁ1.19
sN = 1.08
E6+
E7+
(pcp)2CH+
mfp(Ph)CH+
dfp(Ph)CH+
(mfp)2CH+
dfp(mfp)CH+
(pfp)2CH+
(Ph)2CH+
E8+
E10+
E11+
E12+
E4+
98HFIP2W
96.8% (v/v)
xHFIP = 0.840
N1 = ꢁ1.62
sN = 1.10
E6+
E7+
(pcp)2CH+
mfp(Ph)CH+
(mfp)2CH+
dfp(mfp)CH+
(dfp)2CH+
mfp(Ph)CH+
(mfp)2CH+
dfp(mfp)CH+
(dfp)2CH+
E8+
E11+
E12+
E13+
E8+
99HFIP1W
98.4% (v/v)
xHFIP = 0.914
N1 = ꢁ1.93
sN = 1.09
E11+
E12+
E13+
a) Solvent mixtures are given as w/w. Abbreviations: HFIP = 1,1,1,3,3,3-hexafluoro-2-propanol, W = water. To accommodate
literature conventions in other fields, we also provide the v/v percentages and molar fractions.
b) E parameters derived from kinetic data for reactions of E+ with p-nucleophiles in CH2Cl2; from ref.[18]
c) Laser flash photolysis of triarylphosphonium salts, this work.
d) Calculated from Eqn 1a.
e) Determined from a non-exponential decay curve as the reaction of E1+ with 50HFIP50W does not follow first-order kinetics due to
recombination of E1+ with the photo-leaving group PPh3. See Section S1.3 in the Supporting Information for details.
f) This E parameter is based on only one rate constant.
J. Phys. Org. Chem. 2013, 26 59–63
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J. AMMER AND H. MAYR
(Fig 3b), indicating that the nucleophilicity of pure water is
reduced significantly by the presence of only 10 mol-% of
the good hydrogen bond donor HFIP. A sharp increase between
90 and 100 mol-% water is also found in the ionizing powers
YOTs of the HFIP/water mixtures.[12] These observations are in
agreement with another structural transition of the binary solvent
system, which exists as dilute single HFIP molecules in water
for compositions with more than ~95mol-% water (35HFIP65W,
25 vol.-% HFIP).[24] Large water clusters that are not influenced
by HFIP may also explain why we could not observe E3+ after
irradiation of E3–PAr+3 BF4– in 25HFIP75W (calculated lifetime of
E3+ in pure water: 2.7 ns).
As the nucleophilicity parameters for mixtures with low
water content are highly sensitive to the water content (Fig. 3a),
we did not attempt to determine N1 and sN of pure HFIP.
Literature values of k1 for the decay of E6+ in HFIP range
from “around 102 sꢁ1” (meticulously dried HFIP but inaccurate
measurement)[25] to 2.2 ꢂ 104 sꢁ1 (commercial HFIP, >99.8%,
used as received).[26] The latter value is slightly larger than the
calculated rate constant for the reaction of E6+ with HFIP
containing 1% water (kcalc = 7.2 ꢂ 103 sꢁ1). It thus seems prob-
lematic to derive accurate quantitative information about the
electrophilic reactivities of carbocations from their decay rate
constants in neat HFIP as traces of water may strongly affect
the results.
The N1 parameters for HFIP/water mixtures determined in this
work (Table 2) agree within 1.5 units with our previous estimates
for some of these solvents that were derived from a correlation
with Kevill’s NT parameters.[22] However, the sN parameters of
the HFIP/water mixtures are somewhat larger than for other
alcoholic and aqueous solvents.[22]
The low nucleophilicity of HFIP/water mixtures has previously
been utilized to study solvolysis reactions under conditions
where nucleophilic solvent assistance is reduced. For example,
the solvolysis of 2-propyl tosylate in 97% aqueous HFIP proceeds
by an SN1 mechanism, whereas the same reaction in 97%
aqueous trifluoroethanol already proceeds with measurable
nucleophilic solvent assistance (factor 15).[15] It has been argued
that an intermediate can only exist if its lifetime is at least as long
as the duration of a bond vibration (~10ꢁ13 s).[27–30] Thus, SN1
reactions leading to carbocations with lifetimes shorter than
ca. 10ꢁ13 s cannot occur, and only SN2 reactions are possible in
these cases.
8
7
6
5
4
3
E9+
E6+
E5+
E13+
E1+
E12+
E11+
E10+
E8+
E2+
E3+
E4+ E7+
2
3
4
5
6
7
8
electrophilicity E →
Figure 2. Plot of lg k1 versus E for the reactions of benzhydryl cations
with HFIP/water mixtures (filled symbols). For comparison, data for 100 W
and trifluoroethanol are also shown (open symbols; only a part of the
correlation lines is shown)[22]
a)
10
8
6
4
0
20
40
60
80
100
w.-% H2O
b)
10
8
6
We have previously investigated the borderline between SN1
and SN2 mechanisms for nucleophilic substitution reactions in
DMSO (N1 = 11.3, sN = 0.74).[16] Equation 1a predicts a lifetime
near the theoretical limit (5 ꢂ 10ꢁ14 s) for E9+ (E = 6.70) in DMSO,
and no significant nucleophilic solvent participation was observed
for reactions of the corresponding bromide (i.e., DMSO only
reacts via SN1 and not via SN2 mechanism).[16] On the other hand,
significant nucleophilic solvent participation was found for
reactions of bis[4-(trifluoromethyl)phenyl]methyl bromide,[16] in
agreement with a calculated lifetime of 6 ꢂ 10ꢁ15 s for the
corresponding carbocation (E = 7.96)[18] in DMSO, which is shorter
than the period of a bond vibration (~10ꢁ13 s).
The N1 and sN parameters for the HFIP/water mixtures from
Table 2 can now be employed to predict when a change from
SN1 to SN2 mechanism can be expected in these solvents. The
limiting lifetime of ca. 10ꢁ13 s will thus be reached for carboca-
tions with E ≥ 11 in 50HFIP50W and for carbocations with E ≥ 12
in 70HFIP30W. Carbocations with higher electrophilicity parameters
cannot exist in these solvents. As the decreasing N1 parameters are
4
0
20
40
60
80
100
mol-% H2O
Figure 3. Plots of lg k1 for the decay of E8+ in HFIP/water mixtures
against the water content of the mixtures in weight-% (a) or mol-% (b).
Filled circles: experimental data; open circles: calculated from Eqn 1a
is also found for the ionizing powers YOTs of the HFIP/water
mixtures, which decrease almost linearly with increasing water
content for compositions with 0–90 mol-% water.[12] The inverse
linear correlation between lg k1 (E8+) and YOTs in this range
indicates that nucleophilicity as well as solvent ionizing power of
HFIP/water mixtures are affected in a similar manner by changes
in solvent structure.
Between 90 mol-% water (50HFIP50W) and 100 mol-% water,
however, there is a further substantial increase in nucleophilicity
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Copyright © 2012 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2013, 26 59–63

SOLVENT NUCLEOPHILICITIES OF HEXAFLUOROISOPROPANOL/WATER MIXTURES
compensated by slightly increasing sN parameters in the series
90HFIP10W to 99HFIP1W, we calculate a limiting value of Eꢃ (13
Acknowledgements
We thank Dr. Armin R. Ofial for discussions and the Deutsche
Forschungsgemeinschaft (SFB 749) for financial support.
to 14) for all HFIP/water mixtures with ≤10% water content. These
limiting E parameters are well beyond those of the most electro-
philic carbocations which have so far been characterized by
Eqn 1.[18] It can thus be concluded that nucleophilic solvent
assistance will generally not be observed in HFIP/water mixtures
with ≤10% water content.
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HFIP (Apollo, 99%) was refluxed with CaH2 for 30 min and then distilled
under nitrogen (CAUTION: oil bath temperature≤ 80 ꢀC).[31] Doubly distilled
water (Impendance 18.2 Ω) was prepared with a water purification system
(Milli-Q Plus machine from Millipore). The appropriate amounts of HFIP
and water were combined to obtain the HFIP/water mixtures.
Laser flash photolysis experiments
Solutions of the precursor phosphonium salts E(1–13)–PAr+3 BF4– (Ar = Ph
or p-Cl-C6H4) with A266 nm ꢃ 0.2 to 0.9 (ca. 10ꢁ4 M) were irradiated with a
7-ns laser pulse (fourth harmonic of Nd:YAG laser, lexc = 266 nm, 40–
60 mJ/pulse).[17] Kinetics were measured by following the decay of the
UV/Vis absorbances of the benzhydryl cations in the HFIP/water solvent
mixtures. The rate constants k1 (sꢁ1) were obtained by least-squares
fitting of the absorbance decays of the benzhydryl cations to the single
exponential curve At = A0 eꢁk t + C. Non-exponential kinetics were evaluated
1
using the software Gepasi.[32–35]
[17] J. Ammer, C. F. Sailer, E. Riedle, H. Mayr, J. Am. Chem. Soc. 2012, 134,
11481–11494.
[18] J. Ammer, C. Nolte, H. Mayr, J. Am. Chem. Soc. 2012, 134, 13902–13911.
[19] H. Mayr, T. Bug, M. F. Gotta, N. Hering, B. Irrgang, B. Janker, B. Kempf,
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[20] H. Mayr, B. Kempf, A. R. Ofial, Acc. Chem. Res. 2003, 36, 66–77.
[21] H. Mayr, Angew. Chem. 2011, 123, 3692–3698; Angew. Chem. Int. Ed.
2011, 50, 3612–3618.
CONCLUSION
The excellent linear correlations of lg k1 for the reactions of
benzhydrylium ions with HFIP/water mixtures with the electro-
philicity parameters E of these benzhydrylium ions (Fig. 2) once
again demonstrate that Eqn 1 adequately describes the decay
rate constants of benzhydrylium ions in nucleophilic solvents
and solvent mixtures.[18,22]
In the future, the reactivity parameters of the HFIP/water
mixtures may be of interest for characterizing the electrophilic
reactivities of further highly reactive electrophiles (E > 7) which
cannot be studied in trifluoroethanol or acetonitrile due to
the high nucleophilicity of these solvents.[17] The decay rate
constants of carbocations in many less nucleophilic solvents
such as CH2Cl2, on the other hand, do not reflect the reactivities
of these solvents but result from reactions with impurities or
with the precursor molecules.[17] Mixtures of HFIP and water with
≥90% (w/w) HFIP combine the two advantages that they are less
nucleophilic than trifluoroethanol and acetonitrile but still have a
clearly defined nucleophilic reactivity.
[22] S. Minegishi, S. Kobayashi, H. Mayr, J. Am. Chem. Soc. 2004, 126,
5174–5181.
[23] For example, rate constants k1 ≥ 101 s-1 could be determined for the
first-order decay reactions of benzhydrylium ions in trifluoroethanol:
R. A. McClelland, V. M. Kanagasabapathy, S. Steenken, J. Am. Chem.
Soc. 1988, 110, 6913–6914.
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[26] G. Hallett-Tapley, F. L. Cozens, N. P. Schepp, J. Phys. Org. Chem.
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[28] W. P. Jencks, Chem. Soc. Rev. 1981, 10, 345–375.
[29] J. P. Richard, W. P. Jencks, J. Am. Chem. Soc. 1984, 106, 1373–1383.
[30] J. P. Richard, W. P. Jencks, J. Am. Chem. Soc. 1984, 106, 1383–1396.
[31] T. W. Bentley, C. T. Bowen, W. Parker, C. I. F. Watt, J. Chem. Soc.,
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[32] P. Mendes, Comput. Appl. Biosci. 1993, 9, 563–571.
[33] P. Mendes, Trends Biochem. Sci. 1997, 22, 361–363.
[34] P. Mendes, D. B. Kell, Bioinformatics 1998, 14, 669–883.
[35] Further information about Gepasi: www.gepasi.org
SUPPORTING INFORMATION
Details of the kinetic experiments.
J. Phys. Org. Chem. 2013, 26 59–63
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